Capability indices for non-normal distributions with an application in electrolytic capacitor manufacturing

Abstract

Process capability indices C p( u, v), which include the four basic indices C p, C pk, C pm and C pmk as special cases, have been proposed to measure process potential and performance. C p( u, v) are appropriate indices for processes with normal distributions, but have been shown to be inappropriate for processes with non-normal distributions. In this paper, we first consider two generalizations of C p( u, v), which we refer to as C Np( u, v) and C′ Np( u, v), to cover cases where the underlying distributions may not be normal. Comparisons between C Np( u, v) and C′ Np( u, v) are provided. The results indicated that the generalizations C Np( u, v) are superior to C′ Np( u, v) in measuring process capability. We then present a case study on an aluminum electrolytic-capacitor manufacturing process to illustrate how the generalizations C Np( u, v) may be applied to actual data collected from the factories.